Continuing the refinement of gamma-chymotrypsin, 1.9 A data have been collected and are currently being used in the refinement of the structure by real space electron density fitting. It is anticipated that this refinement will soon be complete, at which point a detailed comparison will be made with the other serine proteases, trypsin, elastase and gamma-chymotrypsin. The acid protease from Rhisopus chinensis: A 3.0 A map has been calculated. The backbone of the molecule has been traced for about 95% of the molecule with confidence. The binding site for inhibitors has been located in a large cleft in the molecule. Data are now being collected to a higher resolution. Several regions of low density, together with the lack of sequence data have prevented the tracing of the entire chain with complete connectivity.